1. Field of the Invention
The present invention relates to a method and a device for ascertaining the degree of compaction in a bed of material with a vibratory compacting device. The invention is particularly related to a method and a device for ascertaining the degree of compaction by evaluating the signals from at least one transducer which is responsive to the vibratory motion of the compacting device as well as that of the bed being compacted.
2. Description of the Prior Art
It was previously known to estimate the degree of compaction achieved with a compacting apparatus by controlling parameters such as vibratory frequency, vibratory amplitude, compaction time etc. in response to the observed degree of compaction. A serious drawback in this context is the difficulty of finding physical characteristics that are easy to measure, exhibit a significant relationship to the degree of compaction achieved and lend themselves to the control of the compacting device.
Natural physical characteristics such as soil density, soil elasticity coefficient etc. are not possible to measure continuously with simple devices. Several proposals have therefore made use of the vibratory motion of the vibratory device and/or that of the soil. By estimating the relationship between the force or the energy developed by the vibrator in the compacting device and the motion of the soil, it is possible to determine the nature of the vibratory motion resistance of the soil.
A device of the last mentioned type is disclosed in British Pat. No. 1372567 published Oct. 30, 1974. In FIGS. 17-19 and the accompanying text in accordance with this previously known device, two acceleration pick-up devices 147 sense the motion of the soil and two strain gauges 158 sense the force of a vibrator 154-156. By comparing the amplitudes of the output signals from the acceleration pick-ups 157 and from the strain gauge 158, it is possible to determine the nature of the vibratory motion resistance of the soil and the result of the comparison can be used for estimating or controlling the operation of the compacting machine.
Another prior art device of a different type for estimating the degree of compaction achieved with a vibratory device is disclosed in U.S. Pat. No. 3,599,543 issued Aug. 17, 1971 to Kerridge. The motion of a vibrating part, for example the roller, of a vibratory roller may in accordance with said U.S. Patent be approximated by an ellipse. From FIG. 1 of said U.S. Patent, it is apparent that the major axis of the ellipse will increase with an increasing number of passes to and fro over the soil. Therefore, the major axis is used as a measure of the degree of compaction. In order to determine the major axis of the ellipse, the vibratory motion of the roller is sensed by a number of accelerometers the positions and orientation of which are clearly indicated in FIGS. 2 to 4 of the said U.S. Patent.
In another prior art device disclosed in U.S. Pat. No. 3,053,157, it is suggested that the degree of compaction achieved with a dynamic soil compacting apparatus be optimized by setting the vibratory frequency at the resonance frequency of the system comprising the dynamic soil compacting apparatus and the soil. To this end, the vertical acceleration of a part of the compacting apparatus is sensed by a transducer and is controlled in response to the output signal of the transducer. However, this device does not provide for any direct measurement or estimation of the achieved degree of compaction.